The invention relates, in general, to a method for producing injection-molded parts, in particular PET preforms.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
An apparatus for producing injection-molded parts, in particular PET preforms, typically includes an extruder for processing and/or plasticizing material that is normally present in granular form. A transfer reservoir receives processed and/or plasticized material, and a piston unit is provided for further processing the material. Normally, the production of PET preforms using an injection molding process involves the application of a single screw extruder for plasticizing materials, such as plastic pellets. For subsequent processing, the plasticized material is injection-molded with a piston unit—shot pot—or directly injected with an axially movable screw. Because PET is hygroscopic and because moisture causes a hydrolytic decomposition of polymer chains, the material is pre-dried in a most complex process, in particular when material in granular form is involved.
Pre-drying involves introduction of air which is heated electrically or with gas burners in a large-volume insulated reservoir. The material is hereby dehumidified and undergoes a temperature increase. In this single-screw plasticization with pre-drying, energy consumption is significantly increased in relation to the temperature increase of the material, typically to 0.55 kWh/kg. In addition, drying plants require significant installation space, which considerably increases the vertical dimensions of the apparatus for producing injection-molded parts. Reference is made in this context to European Pat. No. EP 0 538 286 B1.
Granular and recycled PET can also be processed undried into films and fibers with a starve fed twin-screw extruder by using suitable processing components and a suitable vacuum extraction. This plasticizing approach without pre-drying, is not applicable however in certain fields, like for example PET preforms, because there are concerns with respect to hydrolytic decomposition and a possible increase of the acetaldehyde content—AA content—in the melt. A low M content, however, is an important performance characteristic of preform facilities, because the overwhelming majority of the produced preforms is used for filling water and CSD—Carbonated Soft Drinks. A high AA content hereby adversely affects the taste of the beverages.
Using a twin-screw extruder for producing PET preforms also poses problems with the synchronization of the cyclic operation of an injection molding machine and the continuously operating twin-screw extruder. It has been observed when using twin-screw compounders in an integrated injection compounder that the combination of these machines requires a reservoir between the extruder and the shot pot.
PET preforms can also be produced with injection molding machines equipped with a single screw as the plasticizing unit. PET is hereby pre-dried over several hours to a residual moisture content of about 20 ppm, then plasticized in the single screw, and injection-molded to preforms. If products containing additives must be processed, these additives must be incorporated in an upstream compounding process.
When producing preforms for milk bottles, juice bottles or similar applications, but also various injection-molded parts, these products must have certain barrier characteristics to prevent the content from changing, for example, due to diffusion of content materials out of the injection-molded parts or intake of substances. Such barrier characteristics can be used, for example, as a protection against photo oxidation, UV light or oxygen exposure. Processing such additives with a single screw extruder is presently not possible at all or only with insufficient quality. Therefore, only products can be processed for an application which involves this incorporation in an upstream compounding step. Even incorporating 50% titanium dioxide—TiO2-PET master batches—for achieving a 10% TiO2 fraction in a milk bottle, as required for adequate O2 barrier characteristics, is not possible because of the insufficient mixing effect of a single-screw extruder. Powdered TiO2 cannot be processed with a single-screw extruder, at least not in the required quantity. Another possibility for protecting the content of such injection-molded parts includes application of a barrier layer in an additional processing step. However, this requires additional facilities which is time consuming and therefore expensive.
It would therefore be desirable and advantageous to provide an improved method for producing injection-molded parts, which obviates prior art shortcomings and is able to specifically produce injection-molded parts with various properties in an energy-saving, space-saving, material-protecting and universally applicable manner.